Process and apparatus for flow soldering

ABSTRACT

There is provided a flow soldering process for mounting an electronic component onto a board by means of a lead-free solder material, wherein the molten solder material is supplied and attached to a predetermined portion of the board in a solder material supplying zone, and thereafter the board is cooled by a cooling unit in a cooling zone such that the solder material adhering to the board is rapidly cooled to solidify.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The present application claims a priority under 35 U.S.C. §119 toJapanese Patent Application No. 2000-249588 filed on Aug. 21, 2000,entitled “PROCESS AND APPARATUS FOR FLOW SOLDERING”. The contents ofthat application are incorporated herein by the reference thereto intheir entirety.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to a process and an apparatus for“flow soldering” (which is also called as “wave soldering”) wherein (an)electronic components are mounted onto (or bonded to) a board (or asubstrate) by means of a lead-free solder material.

[0004] 2. Description of Related Art

[0005] In recent years, it has been strongly desired to increase areliability of an electronic circuit board which is contained in anelectronic device with downsizing and a high performance of theelectronic device. Therefore, there is increasing a demand to increase areliability property such as a thermal shock resistance and a mechanicalstrength of a connecting portion which is formed by soldering anelectronic component onto a board in the field of mounting theelectronic components.

[0006] Moreover, while concern about the protection of globalenvironment is increased in a worldwide scale, a regulation or a lawsystem to control industrial waste treatments is being arranged.Although an Sn-Pb based solder material which contains Sn and Pb as maincomponents (e.g. a so-called “63 Sn-37 Pb” eutectic solder material) isgenerally used in an electronic circuit board which is incorporated inan electronic device, lead contained in such solder material may causean environmental pollution if it is subjected to an inadequate wastetreatment, so that researches and developments are carried out as to asolder material which does not contain lead (i.e. a so-called lead-freesolder material) as an alternative of the solder material containinglead.

[0007] A conventional flow soldering process for producing an electroniccircuit board by connecting a electronic component(s) to a board such asa printed board as well as an apparatus for such process will bedescribed with referring to drawings below. FIG. 3 shows a schematicview of the conventional flow soldering apparatus.

[0008] At first, a board is prepared prior to soldering, wherein athrough hole(s) is formed through the board, and an electronic componentis located by inserting a lead (e.g. an electrode) of the component intothe through hole from an upper surface of the board. In such board, aland which is made of copper or the like is formed on a region (A+B+C,see FIG. 4) consisting of (A) a surface which defines the through holeas well as (B) an upper surface portion and (C) a lower surface portionof the board which portions surround the through hole, and such land isconnected to a circuit pattern on the upper surface of the board. On theother hand, regions of the upper surface and the lower surface of theboard but the land(s) are covered by a solder resist.

[0009] Next, the board is subjected to a pre-treatment in which thelower surface of the board on which surface no electronic component islocated is applied with flux by means of a spray fluxer (not shown). Thepre-treatment is conducted in order to improve wetting and spreading ofthe solder material on a surface of the land by removing an oxide film(such as a film formed by natural oxidation) which is inevitably formedon the land.

[0010] Then, with referring to FIG. 3, thus prepared board (not shown)is put into the flow soldering apparatus 60 while said upper surface onwhich the electronic component is located is kept upward (with regardingto the drawing), and the board is mechanically transferred in adirection of an arrow 61 inside the flow soldering apparatus 60 with asubstantially constant velocity by means of a conveyer. In the flowsoldering apparatus 60, the board is first heated in a preheating zoneby means of a preheating unit (or preheater) 62 in order to make theflux applied to the board according to the pre-treatment effectivelydisplay its activity ability.

[0011] Thereafter, when the board is conveyed into a solder materialsupplying zone located above solder wave nozzles 64 and 65, the soldermaterial (not shown) which in a molten state by heating beforehand in asolder material supplying unit 63 is supplied to the board from itslower side through the primary wave nozzle 64 and the secondary wavenozzle 65 in the form of a primary wave and a secondary waverespectively. The solder material thus supplied goes up from the lowersurface of the board by means of the capillary action through an annularspace between the surface of the through hole (i.e. the land) and thelead which is inserted through the through hole from the upper surfaceof the board, and thereafter the solder material naturally cools byreleasing its heat to surrounding of the board with its temperaturedrop, so that the solder material solidifies to form an connectingportion of the solder material (or a so-called “fillet”). In this stepof supplying the solder material (or the step of flow soldering), theprimary wave functions to sufficiently wet the surfaces of the lead andthe land with the solder material, and the secondary wave functions toremove the solder material on regions covered with a solder resist suchthat the solder material does not form a bridge when remaining andsolidifying between the lands (the bridge is not desirable because itcauses a short circuit) and the solder material forms no cornuteprojection, whereby controlling (or conditioning) the form of thefillet.

[0012] As described above, the fillet (or the connecting portion) madeof the solder material is formed which electrically and physically (ormechanically) connects the lead of the electronic component and the landformed in the board.

[0013] The fillet made of the solder material as described above isrequired to have a sufficiently large connecting strength between thelead of the electronic component and the land of the board in order toprovide a high reliability of the electronic circuit board. However,referring to FIG. 4, if the electronic circuit board 70 is produced byusing the lead-free solder material according to the conventional flowsoldering process as described above, the solder material having wettedand spread on the surface of the land 73 (which is located to cover aninside surface which defines the through hole 72 perforated through theboard 71 as well as regions which surround the through hole 73 on theupper side and the lower side of the board 71) partially peels off at aninterface between the solder material and the land as indicated by thearrow 80 upon the solidification of the solder material, so that therearises a problem in that the connection between the land 73 and thefillet 74 made of the solder material becomes insufficient and therebyno high connecting strength between the lead 75 and the land 73 can beobtained.

[0014] Such phenomenon of peeling off of the fillet 74 from the land 73is generally referred to as a “lift-off” phenomenon, which frequentlyoccurs when the lead-free solder material is used though it scarcelyoccurs when the Sn-Pb based solder material is used. The lift-offnotably occurs especially in the case using the lead-free soldermaterial which contains Sn and Bi (such as an Sn-Ag-Bi based material)and the case using the lead-free solder material for connecting a leadwhich is plated with an Sn-Pb based material.

[0015] As a reason for the occurrence of the lift-off, it could begenerally considered that the solder material used for the flowsoldering and/or a metal material which can elute into the soldermaterial upon soldering (e.g. a plating metal for the lead) form a weakalloy of which melting point is lower than that of the initial soldermaterial and of which composition is different from that of the initialsolder material (hereinafter, such alloy is merely referred to as a“low-m.p. alloy”) upon the solidification of the solder material fromits molten state.

[0016] The molten solder material being at a high temperature loses itsheat mainly via the lead 75 which comes from the electronic component(not shown). In the solder material 74 which is supplied and adheres tothe board 71, a temperature gradient is formed by a flux of such heatpassing through the lead 75. The solidification of the solder material74 progresses according to the temperature gradient, wherein the lowesttemperature portion of the solder material 74 is formed at the top ofthe solder material 74 (which is indicated by the arrow 81) and thehighest temperature portion is formed in the vicinity of the interfacebetween the fillet 74 and the land 73 which is a good heat conductor, sothat the solidification begins at the top and ends at the interfacebetween the land 73 and the fillet 74 in due course. On suchsolidification, the low-m.p. alloy as described above is distributed (orsegregated) in a more amount in a still molten portion of the soldermaterial which has not solidified yet, so that the low-m.p. alloy istransferred to and concentrated (or segregated) in the molten portionwith the progress of the solidification. That is, a segregationphenomenon occurs upon solidification of the solder material, and as aresult, the low-m.p. alloy 76 gathers at the interface between the land73 and the fillet 74 where the solidification occurs at the latest. Insuch solidification, the fillet 74 solidifies at the top first andattaches to the lead 75, a tension is generated by the contractionthrough the solidification of the solder material in the direction ofthe arrow 82, and a tension is generated by the thermal contraction ofthe board 71 in the direction of the arrow 83. It could be consideredthat the weak low-m.p. alloy 76 which is enriched in the vicinity of theinterface between the land 73 and the fillet 74 as describe above cannot endure these tensions, so that the peel-off phenomenon is caused atthe interface as shown in the direction of the arrow 80.

SUMMARY OF THE INVENTION

[0017] The present invention has been made for solving the problemsdescribed above and further improving the conventional technique asdescribed above, and the present invention aims to provide a flowsoldering process for mounting (or bonding) an electronic component(s)onto a board by means of a lead-free solder material which process caneffectively reduce occurrence of the lift-off phenomenon, and alsoprovide an apparatus for conducting such process.

[0018] According to one aspect of the present invention, there isprovided a process for mounting an electronic component(s) onto a boardby means of a lead-free solder material (which is also simply referredto as the solder material), which process comprises supplying a melt ofthe solder material in a solder material supplying zone such that thesolder material adheres to a predetermined portion of the board, andthereafter cooling the board by a cooling unit in a cooling zone suchthat the solder material adhering to the board is quickly cooled (orquenched) so as to be solidified.

[0019] In particular, it is sufficient that the cooling of the board(and thus the rapid cooling of the solder material) in the cooling zoneof the present invention is carried out by positively (or forcedly)cooling the board by means of the cooling unit for at least a periodbetween a time at which the solder material in the molten condition hasjust been at a temperature of its melting point and a time at which allof the solder material (or a connecting portion of the solder material)has just completed its solidification. As far as such cooling isensured, any appropriate cooling unit may be used for an appropriatetime including the above period. Further, the cooling of the board inthe cooling zone may be carried out in any appropriate manner as far asthe solder material which is adhering to the board is rapidly cooled bycooling the board positively (or forcedly) by means of the cooling unit.For example, a whole of the board may be cooled or at least a part ofthe board (i.e. only a necessary part) may be cooled.

[0020] According to the present invention as described above, thelead-free solder adhering to the board is quickly cooled (or quenched)by positively cooling the board using the cooling unit, so that a periodfrom the start of the solidification of the solder material to the endof the solidification (i.e. the complete solidification) is shortened.In the conventional process, the segregation phenomenon occurs in thesolder material by the temperature gradient within the inside of thesolder material (or the fillet) since the board is not positivelycooled, so that the weak low-m.p. alloy finally gathers in the vicinityof the interface between the fillet and the land as described above. Onthe other hand, in the present invention, the solder completelysolidifies in a shorter period in the present invention compared withthe conventional process, so that the segregation phenomenon can bealleviated, and in particular gathering of the low-m.p. alloy in thevicinity of the interface is avoidable so that the occurrence of thelift-off phenomenon is suppressed.

[0021] Furthermore, in the conventional process, a metal phase which ishard and brittle (such as a Bi phase or a Bi mass) is formed in thefillet on solidifying of the solder material and thereby providesbrittleness to the fillet, so that the fillet has a low mechanicalstrength. According to the present invention, on the other hand, theperiod required for the solidification of the solder material isshortened, so that the period during which such metal phase grows isshortened, which makes a structure of the metal phase in the filletminute (or fine). As a result, the mechanical strength of the fillet isincreased by the present invention.

[0022] In a preferred embodiment, the lead-free solder material iscooled in the cooling zone with at a cooling rate of at least 200°C./min. The cooling rate is preferably as large as possible, and thecooling rate is, for example, in a range of about 200 to 500° C./min,and preferably about 300 to 500° C.

[0023] It is noted that the cooling rate is referred to as a rate oftemperature decreasing of a solder material portion (and in particular,a portion to be a connecting portion (or fillet portion) made of thesolder material by solidification of the solder material), and it isinherently an average temperature decreasing rate over a period betweena time at which the solder material in the molten condition has justbeen at a temperature of its melting point and a time at which thesolder material (or a whole of the solder material portion) has justcomplete solidifying. However, there is practically no problem when suchaverage temperature decreasing rate is regarded as an average of ameasured temperature of the solder material portion at a moment when theboard gets into the cooling zone and a measured temperature of thesolder material portion at a moment when the board gets out of thecooling zone, and thus such average of the measured temperatures isconveniently used as the average temperature decreasing rate (i.e. thecooling rate) in the present specification. The temperature of thesolder material portion (i.e. connecting portion of the solder material)can be measured by a thermocouple placed on (for example, attached to) apredetermined portion of the board on which the solder material is toadhere (for example, a land locating on one side of the board to whichthe solder material is supplied), and recording data from thethermocouple together with time.

[0024] The positive cooling of the board in the cooling zone asdescribed above can be carried out by gas cooling or liquid cooling, andas the cooling unit for cooling the board, a unit which employs the gascooling or the liquid cooling can be used. It is noted that the gascooling means cooling which uses a gas (such as air (or ambient air), aninert gas such as nitrogen gas) as a coolant of which temperature islower than a temperature of the board (and in particular that of thesolder material which is adhering to the board), and generallyconsiderably lower than such temperature. The gas cooling allows theboard to be in contact with such gas so as to cool the board. Forexample, such gas has a temperature in the range between −20 and 30° C.,and preferably in the range between −10 and 10° C. The gas cooling canbe carried out by passing the board through an atmosphere including suchgas or by blowing such gas directly toward the board. Also, the liquidcooling means cooling which uses a liquid (such as water, liquefiednitrogen or other liquid) as a coolant of which temperature is lowerthan a temperature of the board (and in particular that of the soldermaterial which is adhering to the board), and generally considerablylower than such temperature. The liquid cooling allows at least anecessary portion or a whole of the board to be in contact with suchliquid so as to cool the board. For example, such liquid has atemperature in the range between 1 and 30° C., and preferably in therange between 1 and 10° C. The liquid cooling can be carried out bydipping the board into an amount of the liquid or by spraying the liquidtoward the board. The liquid cooling can use a latent heat ofvaporization of the liquid, which is advantageous for the readily rapidcooling.

[0025] Concretely, as the cooling unit using the gas cooling, a unit maybe used which comprises a blower or a gas flowing device (including forexample a pump) which supplies the low temperature gas into the coolingzone, and discharges the gas from the cooling zone. Thus supplied gasforms an low temperature atmosphere through which the board is passed.Further, a unit comprising a nozzle(s), a fan, a spot cooler or the likewhich blows the low temperature gas directly toward the board may alsobe used. As the cooling unit using the liquid cooling, a unit may beused which comprises a bath containing the liquid into which the boardis dipped so as to cool an entire of the board, an atomizer which formsand sprays a mist of the liquid toward at least a portion of the board,a nozzle which supplies a relatively small amount of the liquid to atleast a portion if the board or the like. Depending on a kind of suchcooling unit, it may be located in the cooling chamber or the unit isconnected to the cooling chamber so that a portion or a whole of theunit is located outside the cooling chamber. It is of course possible toemploy any combination of the above mentioned various kinds of thecooling units. In order to obtain a larger cooling rate, it ispreferable that the liquid cooling rather than the gas cooling isemployed for the purpose of the positive cooling.

[0026] The cooling rate can be obtained as desired, in the case of thegas cooling, by controlling a temperature of the gas which is suppliedto the cooling zone and a flow rate of such gas, a transfer speed of theboard and so on while considering heat capacities of the board and thecomponent(s) which is located on the board. In the case of liquidcooling, a desired cooling temperature can be obtained by controlling atemperature of the liquid which is contacted with the board, thetransfer speed of the board, and so on while considering the heatcapacities of the board and the component(s) which is located on theboard. For instance, a cooling rate of about 210° C./min. is obtained bypassing a board made of a glass epoxy resin and having a size of200mm×200 mm×0.8 mm at a speed of about 1.2 m/min. through a chamberhaving a size of 1000 mm×500 mm×500 mm while blowing a gaseousatmosphere having a temperature of about 25° C. towards the board at aflow rate of about 2 liter/ min.

[0027] The positive cooling of the board in the cooling zone ispreferably conducted by the gas cooling in which the cooling unit usesnitrogen gas. In the gas cooling using nitrogen gas, oxidation of thesolder material can be prevented so that a wetting property of thesolder material (and especially the wetting property with respect to theland) is improved, which further decreases the occurrence of thelift-off.

[0028] In a preferred embodiment, the process of the present inventionfurther comprises, after supplying the molten solder material to theboard in the solder supplying zone so as to adhere to the predeterminedportion of the board and before cooling the board in the cooling zone,locating the board in a conditioning zone having an atmosphere at atemperature which ensures a complete molten state of the solder materialadhering to the board. It is sufficient that the complete molten stateof the solder material is ensured at least at a time just before theboard is cooled in the cooling zone. It is readily understood by thoseskilled in the art that such conditioning zone is located downstream ofthe solder material supplying zone and upstream of the cooling zone withrespect to the transfer direction of the board.

[0029] The conditioning zone functions to condition the board after thesolder material is attached to the board and before the board is rapidlycooled. In particular, it conditions the board (particularly the soldermaterial adhering to the board) such that the solder material at anyposition of the board is substantially entirely in the molten state(that is, the entire of the solder material is substantially in themolten state homogeneously over an entire of the board). Concretely,when the board comes into the conditioning zone while a portion of thesolder material which is adhering to the board is naturally cooled tostart its solidification, the board (and thus the solder material) isheated in the conditioning zone so that the entire of the soldermaterial which is solidifying is again melted preferably completelybefore the board is cooled in the cooling zone. When the board comesinto the conditioning zone while substantially the entire of the soldermaterial which is adhering to the board is in the molten condition, theboard is located in an atmosphere of the conditioning zone in which theboard (and thus the solder material) does not lose an amount of heatexcessively (preferably by being supplied with heat) so that the soldermaterial does not start its solidification at least before the boardgoes into the cooling zone, whereby the molten state of all of thesolder material can be kept. Thus, in this case, no positive heating ofthe board is necessarily carried out. The conditioning zone may bereferred to as a heating zone which heats the board by supplying anamount of heat to the board so as to the temperature of the soldermaterial is raised, or a constant temperature zone which is kept at acertain temperature and in which an excessive temperature drop (and thusexcessive cooling) of the solder material (thus the board) is suppressedduring the board passes through the conditioning zone (or until it comesinto the cooling zone) by supplying a less amount of heat or thermallyinsulating the conditioning chamber.

[0030] The provision of the board in the conditioning zone as describedabove can reduce a temperature difference (or narrow a temperaturedistribution) within the solder material of each connecting portion aswell as a temperature difference within connecting portions of thesolder material over the board as a whole. In other words, theconditioning zone functions to make the board and thus the soldermaterial condition thermally uniform. In the absence of the conditioningzone, the solder material may begin to partly solidify before the boardis cooled in the cooling zone, and in such case, an initial temperatureupon starting rapidly cooling in the cooling zone may vary within asingle connecting portion of the soldering material or throughout all ofthe connecting portions to some extent. On the other hand, when theboard is conditioned to be thermally uniform beforehand by providingsuch conditioning zone upstream of the cooling zone, the variation ofthe initial temperature upon rapidly cooling, and thereby variation of aperiod required for the solidification of the solder material can besuppressed. As a result, it is possible to further decrease theoccurrence ratio of the lift-off.

[0031] The temperature of the atmosphere of the conditioning zone asdescribed above can be any temperature as long as it ensures the moltencondition of the solder material before the board goes into the coolingzone and it is less than a heat resistant temperature of the electroniccomponent mounted on the board so as to avoid thermally damage of theelectronic component. The temperature of the conditioning zone ispreferably in the range which is not less than the melting point of thesolder material and which is less than the heat resistant temperature ofthe electronic component in order to completely or entirely melt thesolder material. More preferably, the temperature of the conditioningzone is within the range which is higher than the melting point of thesolder material by 10° C. and lower than the heat resistant temperatureof the electronic component by 5° C. However, the present invention isnot limited to the above. For example, where the solder material whichis adhering to the board is at a higher temperature than the meltingpoint of the solder material (i.e. the solder material has not startedits solidification) when the board comes into the conditioning zone, andthe conditioning zone can prevent such excessive temperature decrease ofthe solder material therein that the solder material starts to solidifyduring the board passes through the conditioning zone, the temperatureof the conditioning zone is not necessarily higher than the meltingpoint of the solder material. It is of course preferable even in suchcase that the temperature of the conditioning zone is not lower than themelting point of the solder material and lower than the heat resistanttemperature of the electronic component from a viewpoint of the completemolten condition of the solder material.

[0032] It is preferable that the atmosphere of the conditioning zoneconsists essentially of nitrogen gas. As a result, the oxidation of thesolder material and the land is prevented to avoid degradation of thewetting property of the solder material, so that a connecting area ofthe land with the solder material is sufficiently ensured to suppressthe peel-off of the solder material.

[0033] The atmospheres of the conditioning zone and the cooling zone canbe selected independently of each other, though both of them arepreferably of nitrogen gas.

[0034] According to other aspect of the present invention, there isprovided an apparatus for mounting (or bonding) an electroniccomponent(s) onto a board by a flow soldering process using a lead-freesolder material (which is also referred to as merely “solder material”as described above), which apparatus comprises a solder materialsupplying unit (or a solder material supplier) which supplies a moltensolder material so as to attach it to a predetermined portion of a boardlocated in a solder material supplying chamber and a cooling chamberlocated downstream of the solder material supplying chamber, wherein theboard is cooled by a cooling unit (or a cooler) so as to rapidly cooland solidify the solder material which is adhering to the board.

[0035] In a preferred embodiment, the board is cooled by the coolingunit in the cooling chamber such that the lead-free solder material israpidly cooled at the cooling rate which is not less than 200° C./min.As the cooling unit, a unit may be used which uses the gas cooling orthe liquid cooling, and preferably the gas cooling with nitrogen gas asexplained in the above with reference to the flow soldering processaccording to the present invention.

[0036] In a preferred embodiment, the apparatus of the present inventionfurther comprises a conditioning chamber between the solder materialsupplying chamber and the cooling chamber with respect to the transferdirection of the board. An atmosphere in the conditioning chamber has atemperature at which the solder material is in the completely moltencondition is ensured. The temperature of such atmosphere in theconditioning chamber is preferably not lower than the melting point ofthe solder material and lower than the heat resistant temperature of theelectronic component. Further, the conditioning chamber preferablycontains a nitrogen gas atmosphere.

[0037] It is noted that the term “chamber” is intended to mean astructural member which defines a space, the term “zone” is intended tomean a space (or a spatial member) within the chamber, and the term“atmosphere” is intended to mean a gaseous atmosphere (or a gas) in thespace formed by the chamber (thus, an atmosphere in a zone issubstantially the same as an atmosphere in a chamber). For example, the“solder material supplying chamber” defines the “solder supplying zone”of the flow soldering process according to the present invention, andthis is similarly applicable to the “cooling chamber” and the“conditioning chamber”.

[0038] The process according to the present invention as described aboveis conveniently carried out using the apparatus according to the presentinvention. Therefore, it is understood by those skilled in the art thatthe above descriptions as to the solder material supplying zone and thecooling zone as well as the optional conditioning zone with reference tothe preferable embodiments of the process according to the presentinvention are also applicable to the solder material supplying chamberand the cooling chamber as well as the optional conditioning chamber.

[0039] In the flow soldering apparatus according to the presentinvention, the solder material supplying chamber does not have to beformed such that the solder material supplying zone is definitelydivided from other spaces in the apparatus. The cooling chamber maydefinitely divide the solder material supplying zone from other spaceshaving relatively high temperatures at least to such an extent that thepositive cooling of the board is effectively carried out in theapparatus of the present invention. Also, the conditioning chamber maybe such that its atmosphere is definitely divided from other atmosphereshaving relatively high temperatures (such as an atmosphere of the soldermaterial supplying chamber), but not necessarily definitely divided fromother spaces in the apparatus. However, the conditioning chamber and thecooling chamber are preferably structured such that atmospheres of thesechambers are divided from each other to some extent from a viewpoint ofthe thermal efficiency.

[0040] It should be noted that the solder material supplying chamber,the cooling chamber and the optional conditioning chamber which aredescribed above with reference to the flow soldering apparatus accordingto the present invention do not necessarily have to be used uponconducting the flow soldering process according to the presentinvention, and other apparatus may be used as far as it allows the rapidcooling as described above.

[0041] The lead-free solder material which can used for the flowsoldering process and/or the flow soldering apparatus according to thepresent invention includes, for example, an Sn-Cu based material, anSn-Ag-Cu based material, an Sn-Ag based material, an Sn-Ag-Bi basedmaterial, an Sn-Ag-Bi-Cu based material and the like. Regarding to theboard, a board made of, for example, a paper phenol, a glass epoxyresin, a polyimide film, a ceramic or the like can be used. Theelectronic component connected to the board can be any electroniccomponent which is connected to the board by means of the through holeformed through the board, for example, a DIP IC (Dual In-linePackage-integrated Circuit), a connector and an axial lead component.However, these are described only to the illustrative purpose and thepresent invention is not limited thereto.

BRIEF DESCRIPTION OF THE DRAWINGS

[0042] More sufficient appreciation of the invention and many of theattendant advantages thereof will readily become apparent with referenceto the following detailed description, particularly when considered inconjunction with the accompanying drawings, in which:

[0043]FIG. 1(a) shows a schematic view of a flow soldering apparatus inone embodiment of the present invention;

[0044]FIG. 1(b) shows a schematic partial view of a flow solderingapparatus of FIG. 1(a) in which an inside of a cooling chamber is shown;

[0045]FIG. 2 shows a schematic view of a flow soldering apparatus inother embodiment of the present invention;

[0046]FIG. 3 shows a schematic view of a conventional flow solderingapparatus; and

[0047]FIG. 4 shows a schematic cross sectional view of a part of anelectronic circuit board which is produced according to a conventionalflow soldering process.

[0048] Numerals in the drawings denote the following elements:

[0049]1 . . . arrow (transfer direction of board)

[0050]2 . . . preheating unit;

[0051]3 . . . solder material supplying unit;

[0052]4 . . . primary wave nozzle;

[0053]5 . . . secondary wave nozzle;

[0054]6 . . . solder material supplying zone;

[0055]7 . . . cooling zone;

[0056]8 . . . conditioning zone;

[0057]9 . . . preheating zone;

[0058]12 . . . nozzle (a part of cooling unit);

[0059]13 . . . board;

[0060]16 . . . solder material supplying chamber;

[0061]17 . . . cooling chamber;

[0062]18 . . . conditioning chamber;

[0063]19 . . . preheating chamber; and

[0064]20 and 30 . . . flow soldering apparatus.

DETAILED DESCRIPTION OF THE INVENTION

[0065] Hereinafter, two embodiments of the present invention will bedescribed in detail with referring to the drawings.

First Embodiment

[0066]1A flow soldering process in the first embodiment of the presentinvention is conducted by using a flow soldering apparatus of thepresent invention as shown in FIG. 1(a).

[0067] Referring to FIG. 1 (a), the flow soldering apparatus 20 of thepresent embodiment comprises a preheating unit (or a preheater) 2, asolder material supplying unit 3 which supplies a melt of the soldermaterial to the board (not shown in FIG. 1(a)), a preheating chamber 19,a solder material supplying chamber 16, and a cooling chamber 17 throughwhich chambers 19, 16 and 17 the board(s) is passed in a direction ofthe arrow 1. The solder material supplying unit 3 contains the soldermaterial therein (not shown) which has been heated to melt beforehand(i.e. the molten solder material) and which supplies, to the board inthe solder material supplying chamber 16 (and thus in the soldermaterial supplying zone 6), the solder material in the form of waves byflowing the solder material through the primary wave nozzle 4 and thesecondary wave nozzle 5. It is noted that each of the preheating chamber19, the solder material supplying chamber 16 and the cooling chamber 17has an inner space as the preheating zone 9, the solder materialsupplying zone 6 and the cooling zone 7 respectively, and there aregenerally provided passages between them so that the board can passthrough these zone. As shown in the embodiment of FIG. 1(a), there maybe no clear border between the preheating chamber 19 and the soldermaterial supplying chamber 16, which will be explained below.

[0068] The preheating chamber 19 defines the preheating zone 9 in whichthe board is preheated. The preheating unit 2 is contained in thepreheating chamber 19 in the shown embodiment, through the presentinvention is not limited to such embodiment. The preheating unit 2 maybe outside the preheating chamber 19. The preheating unit is notnecessarily required upon conducting the present invention, but it ispreferable that the preheating unit 2 is provided. The solder materialsupplying chamber 16 is located above the solder material supplying unit3 and defines the solder material supplying zone 6 in which the soldermaterial in the molten state is supplied to the board through the solderwave nozzles 4 and 5. The preheating zone 9 and the solder materialsupplying zone 6 as described above preferably have high temperatureatmospheres respectively, so that they do not necessarily have to beformed definitely separately. Thus, the preheating chamber 19 and thesolder material supplying chamber 16 (and thus the atmospheres therein)are not necessarily divided distinctly. In the shown embodiment, thepreheating chamber 19 and the solder material supplying chamber 16 areconnected without any partition, and thus the respective atmospherestherein are also connected. However, the present invention is notlimited to such embodiment the preheating chamber 19 and the soldermaterial supplying chamber 16 may be formed such that the respectiveatmospheres therein are separated to some extent.

[0069] The cooling chamber 17 defines the cooling zone 7 whichpositively or forcedly cools the board by means of a cooling unit so asto rapidly cool and solidify the solder material which is adhering tothe board. For such cooling, the cooling chamber 17 comprises thecooling unit (not shown) which can cool the board in the cooling zone 7as its inner space of the cooling chamber 17. The cooling unit may becontained in or connected to the cooling chamber 17. As the coolingunit, any appropriate device can be used as far as the solder materialis rapidly cooled (or quenched). For example, a unit comprising anozzle(s) which ejects a gas as a coolant may be used. Concretely,referring to FIG. 1 (b) which schematically shows an enlarged side viewof the board 13 inside the cooling chamber 17, it is possible to use aunit which is provided with a plurality of nozzles 12 arranged above andbelow a transfer line (shown with the broken line) of the board 13 andwhich is structured such that the gas is ejected as shown with arrowsthrough the nozzles 12 from its source outside the cooling chamber 17 bymeans of a pump (or a blower) also outside the cooling chamber 17. Thenozzles 12 preferably eject a low temperature gas toward the board 13 sothat the board 13 is cooled by the gas cooling. The location of thenozzle(s) 12 in the cooling chamber 17 is not particularly limited asfar as the board 13 in the chamber 17 is cooled so as to rapidly cool tosolidify the solder material entirely. The nozzles are not alwaysprovided on the both sides of the transfer line, and it may be providedon one side. The cooling unit is not limited to that having thenozzle(s) as described above, and other cooling unit may be used whichallows a gas or a liquid or any combination thereof to contact with theboard so as to cool the board, whereby the solder material is rapidlycooled.

[0070] For example, the cooling unit which can be used for cooling theboard may supply the low temperature gas into the cooling chamber 17 sothat the temperature of the atmosphere of the cooling zone 7 is kept ata considerably low temperature (such as a temperature in the rangebetween −20 and 30° C.), whereby the board (more particularly the soldermaterial adhering to the board) is rapidly cooled. In this case, the lowtemperature gas may not have to be directed to the board.

[0071] It is sufficient from a viewpoint of the thermal efficiency thatthe solder material supplying chamber 16 and the cooling chamber 17 asdescribed above define atmospheres therein at least to a degree thatenough wetting up of the solder material on the through hole surface andcooling of the board are effectively provided in these chambers,respectively. It should be noted that the atmospheres in the soldermaterial supplying chamber 16 and the cooling chamber 17 do not have tobe necessarily divided distinctly.

[0072] Then, the flow soldering process according to the presentinvention using the flow soldering apparatus 20 in FIG. 1(a) will bedescribed.

[0073] The board is first subjected to the pre-treatment by applying aflux to a lower surface of the board by means of a spray fluxer. Anelectronic component(s) is located on such board beforehand with alead(s) of the electronic component inserted through a through hole froman upper surface side of the board.

[0074] Such board is put into the flow soldering apparatus 20 in FIG.1(a) while the upper surface on which the electronic component islocated is set upward (with regarding to the drawing), and mechanicallytransferred through the preheating chamber 19, the soldering materialsupplying chamber 16 and the cooling chamber 17 of the flow solderingapparatus 20 in a direction of the arrow 1 along the transfer line(shown with the broken line) with a substantially constant speed bymeans of a conveyer. In the flow soldering apparatus 20, the board isheated (or preheated) to about 150 to 160° C. by the preheating unit 2in the preheating zone 9 of which atmosphere is of atmospheric air andpreferably nitrogen gas. When the board is conveyed into the soldermaterial supplying zone 6 in due course, the solder material (not shown)which has been previously heated and melted is supplied from theunderside of the board through the primary wave nozzle 4 and thesecondary wave nozzle 5 to the board in the form of the primary andsecondary waves respectively by means of the solder material supplyingunit 3. As will be under stood, such supply of the solder material issubstantially the same as the aforementioned flow soldering processwhich is described as the conventional process.

[0075] Following the attachment of the molten solder material to thepredetermined portion of the board as described above, the board islocated in the cooling zone 7 while being transferred into the coolingchamber 17 from the solder material supplying chamber 16. In the coolingzone 7, the board is rapidly cooled with the nitrogen gas cooling byblowing the nitrogen gas having a low temperature toward the board suchthat the solder material is rapidly cooled to solidify. The cooling rateof the solder material is preferably not less than 200° C./min. and itcan be, for example, about 200 to 500° C./min and preferably about 300to 500° C. It is noted that the board can be cooled by means of theother gas cooling (such as air cooling which blows low temperature air(atmospheric air) toward the board) or the liquid cooling (such as watercooling in which a circulated water having a low temperature contactsthe board) while using any appropriate cooling units as described above.

[0076] As the result of the positive cooling of the board while usingthe cooling unit as described above, the solder material solidifies toform a fillet which electrically and physically connects the lead of theelectronic component with the land formed on the board. Thus producedelectronic circuit board is further transferred to the outside of thecooling chamber 17 so as to remove it from the apparatus 20. Asdescribed above, the electronic circuit board in which the electroniccomponent(s) has been soldered to the board can be produced.

[0077] According to the above embodiment, an occurrence of the lift-offphenomenon is effectively suppressed since the period for solidifying ofthe solder material is shortened. Furthermore, a mechanical strength ofthe connecting portion consisting of the solder material (i.e. thefillet) is increased since finer metal phases are formed in thesolidified solder material (i.e. the fillet).

Second Embodiment

[0078] The second embodiment of the present invention will be described.A flow soldering process in this embodiment is conducted by using a flowsoldering apparatus 30 according to the present invention as shown inFIG. 2. In the following description with respect to the secondembodiment, the description which is substantially the same as that ofthe aforementioned first embodiment will be omitted.

[0079] The flow soldering apparatus 30 is similar to the flow solderingapparatus 20 shown in FIG. 1(a) described in connection with the firstembodiment except that the flow soldering apparatus 20 in the firstembodiment is modified to be provided with a conditioning chamber 18,which results in the second embodiment. The conditioning chamber 18forms a conditioning zone 8. Similarly to the solder material supplyingchamber 16, it is sufficient from a viewpoint of thermal efficiency thatthe conditioning chamber 18 defines an atmosphere therein at least tosuch degree that the board is effectively heated or optionally heat lossof the board is effectively prevented. It should be noted that theatmosphere of the conditioning chamber 16 is not necessarily completelyseparated from other atmosphere having a high temperature such as theatmosphere of the solder material supplying chamber 16. In the shownembodiment, the atmosphere of the conditioning chamber 18 is separatedfrom that of the solder material supplying chamber 16 to some extent,but these two chambers are optionally integrated to form one chamberwith taking the thermal efficiency into consideration.

[0080] In the flow soldering process using such flow soldering apparatus30, the board is located in the conditioning zone 8 by transferring fromthe solder material supplying chamber 16 to the conditioning chamber 18following the supply of the solder material to the board in the soldermaterial supplying zone 6 similarly to the first embodiment. Theconditioning zone 8 has an atmosphere of which temperature is preferablynot less than the melting point of the solder material and less than theheat resistant temperature of the electronic component, e.g. about 220to 230° C., in order to ensure that the solder material adhering to theboard is in the completely molten condition. The atmosphere of theconditioning zone may be of air and preferably nitrogen gas.

[0081] Thereafter, the board is transferred from the conditioningchamber 18 into the cooling chamber 17 to be placed in the cooling zone7 where it is cooled as described in the first embodiment. As a result,the solder material solidifies to form the fillet so that the lead ofthe electronic component and the land formed on the board iselectrically and physically connected by the fillet. Thus, theelectronic circuit board in which the electronic component(s) has beensoldered to the board can be produced.

[0082] According to the second embodiment, the temperature difference inthe solder material within the respective connecting portions which arebeing solidified and also the temperature difference in the soldermaterial over a whole of the board which is being solidified (i.e. theoverall temperature difference of the solder material) are diminished(or narrowed), so that the variation of the initial temperature of thesolder material upon the start of the rapid cooling as well as thevariation of the period required for the complete solidification of thesolder material can be made smaller. Therefore, an occurrence of thelift-off phenomenon is further suppressed.

EXAMPLES

[0083] In the flow soldering process, an influence of the cooling rateof the solder material upon the occurrence of the lift-off phenomenonwas examined. Electronic circuit boards were produced under variousconditions of the flow soldering process while using the flow solderingapparatus which is explained above with reference to the first or secondembodiment.

[0084] The used various conditions are listed in the Table 1 below.Referring to Table 1, Examples 1 and 2 which used the cooling zonewithout a conditioning zone correspond to the flow soldering process andapparatus of the first embodiment. On the other hand, Examples 3 to 8which used both of the cooling zone and the conditioning zone (i.e. thecooling chamber and the conditioning chamber) correspond to the flowsoldering process and apparatus of the second embodiment. In all ofExamples, an Sn—-Ag—Bi based material (melting point (m.p.): about 215°C.) was used as the lead-free solder material. TABLE 1 ConditionsExample conditioning zone cooling zone** No. atmosphere temp. atmosphere 1* — — air 3 — — air  2* air 150° C. N₂ gas 4 N₂ gas 150° C. N₂ gas 5air 220° C. air 6 N₂ gas 220° C. N₂ gas 7 air 240° C. air 8 N₂ gas 240°C. N₂ gas

[0085] Electronic circuit boards were produced by conducting the flowsoldering while blowing the gas having a predetermined temperature witha predetermined flow rate so as to achieve the cooling rates of thesolder material of 50, 100, 200, 300, 400 and 500° C./min. respectivelyin Example 1 to 8, and the occurrence ratios of the lift-off phenomenonupon the production of the electronic circuit boards were obtained. Theresults are shown in Table 2. TABLE 2 Occurrence Ratio of Lift-offPhenomenon (%) Example cooling rate (° C./min.) No. 50 100 200 300 400500 1 50 30 15 10 8 5 2 35 18 8 5 3 1 3 43 24 9 7 5 3 4 34 18 7 5 3 1 542 23 8 6 4 2 6 32 16 6 4 2 1 7 41 22 8 5 4 1 8 30 15 5 3 1 0

[0086] As seen from Table 2, the occurrence ratio of the lift-offphenomenon was small as the cooling rate increased as to all theconditions. It was confirmed that the occurrence ratio of the lift-offphenomenon is effectively reduced upon rapidly cooling the soldermaterial as in the flow soldering process of the present invention, andespecially in the case where the cooling rate of the solder material isnot less than 200° C./min. For example, even in Example 1 using thecooling zone alone without a conditioning zone, the occurrence ratio ofthe lift-off phenomenon was reduced to or below 15% by using the coolingrate of the solder material at about 200° C./min. or more.

[0087] Further, referring to Table 2 and comparing the occurrence ratioof Example 1 using the cooling zone alone with those of Examples 3, 5and 7 using both of the cooling zone and the conditioning zone (of whichtemperatures in the conditioning zone are 150, 220 and 240° C.respectively), it can be understood that the occurrence ratios ofExamples 3, 5 and 7 were smaller than that of Example 1, all Examplesbeing conducted the flow soldering in the air. It could be expected thatin Example 3 of which atmosphere temperature of the conditioning zone islower that the melting point of the used solder material (about 215° C.)among Examples 3, 5 and 7, the solder material which was adhering to theboard did not start to solidify so that it was kept substantially in themolten condition before leaving the conditioning zone. It should benoted that when the temperature of the atmosphere of the conditioningzone is lower than the melting point of the solder material, whether thesolder material which is adhering to the board is kept in the moltencondition or not depends on for example the temperature of the soldermaterial which is to be supplied to the board, a speed at which theboard is transferred, a length of the conditioning zone and so on, sothat the atmosphere temperature of 150° C. of the conditioning zonecannot always keep the solder material in the molten condition.Additionally, when comparing the results of Examples 3, 5 and 7, theoccurrence ratio of the lift-off phenomenon was smaller as thetemperature of the conditioning zone atmosphere increased. Especially inthe case of Examples 5 and 7 in which the temperatures of theconditioning zone were higher than the melting point of the Sn-Ag-Bibased material as the lead-free solder material (i.e. 215° C.), theoccurrence ratio of the lift-off phenomenon was reduced to or below 8%by the cooling rate of the solder material at about 200° C./min. ormore. This could be because passing through the conditioning zone ofwhich atmosphere temperature is not less than the melting point of thesolder material ensures that the solder material is kept in the moltencondition.

[0088] Furthermore, it can be understood that the occurrence ratios ofExamples 2, 4, 6 and 8 were smaller than those of Examples 1, 3, 5 and 7respectively, wherein conditions of Examples 2, 4, 6 and 8 weresubstantially same as those of Examples 1, 3, 5 and 7 respectively,except for the atmosphere in which the flow soldering was carried out(i.e. air vs. nitrogen). In other words, the occurrence ratio of thelift-off phenomenon can be further reduced by conducting the flowsoldering in the nitrogen gas atmosphere compared with conducting theflow soldering in the air.

[0089] According to the present invention, there is provided the processof flow soldering in which the lead free solder material is used andthrough which the occurrence of the lift-off is effectively suppressed.Also, there is provided the apparatus for conducting such process.

What is claimed is:
 1. A flow soldering process for mounting anelectronic component onto a board by means of a lead-free soldermaterial, which process comprises: supplying a melt of the soldermaterial such that the solder material adheres to a predeterminedportion of the board in a solder material supplying zone; and thencooling the board by a cooling unit in a cooling zone such that thesolder material adhering to the board is rapidly cooled to solidify. 2.The process according to claim 1, wherein the solder material is cooledby the cooling unit at a cooling rate which is not less than 200°C./min.
 3. The process according to claim 1, wherein the cooling unit isa unit which uses gas cooling or liquid cooling.
 4. The processaccording to claim 3, wherein the cooling unit is a unit which uses thegas cooling with nitrogen gas.
 5. The process according to claim 1,which process further comprises: conditioning the board in aconditioning zone which ensures that the solder material adhering to theboard is in a completely molted condition, after supplying the melt ofthe solder material to the board in the solder material supplying zoneand before cooling the board in the cooling zone.
 6. The processaccording to claim 5, wherein a temperature of the conditioning zone isin the range from a melting point of the solder material to a heatresistant temperature of the electronic component.
 7. The processaccording to claim 5, wherein the conditioning zone has a nitrogen gasatmosphere.
 8. The process according to claim 1, wherein occurrence of asegregation phenomenon in the solder material is suppressed by the rapidcooling of the solder material.
 9. The process according to claim 1,wherein a fine structure of a metal phase is formed by the rapid coolingof the solder material.
 10. An apparatus for mounting an electroniccomponent onto a board through a lead-free solder material by means of aflow soldering process while transferring the board, which apparatuscomprises: a solder material supplying chamber in which a melt of thesolder material is supplied to the board by a solder material supplyingunit such that the solder material adheres to a predetermined portion ofthe board; and a cooling chamber in which the board is cooled by acooling unit such that the solder material adhering to the board israpidly cooled to solidify.
 11. The apparatus according to claim 10,wherein the cooling unit is operated such that the solder material iscooled at a cooling rate which is not less than 200° C./min.
 12. Theapparatus according to claim 10, wherein the cooling unit uses gascooling or liquid cooling.
 13. The apparatus according to claim 12,wherein the cooling unit uses the gas cooling with nitrogen gas.
 14. Theapparatus according to claim 10, which apparatus further comprises aconditioning chamber between the solder material supplying chamber andthe cooling chamber, in which conditioning chamber the board isconditioned such that the solder material adhering to the board in acompletely molten condition is ensured at least before the rapid coolingof the solder material.
 15. The apparatus according to claim 14, whereina temperature of an atmosphere in the conditioning chamber is in therange between a melting point of the solder material and a heatresistant temperature of the electronic component.
 16. The apparatusaccording to claim 14, wherein the conditioning chamber contains anatmosphere of nitrogen gas.